1. Field of the Invention
This invention relates generally to the field of surveying equipment and more particularly to a laser based distance measuring system.
2. Description of the Related Art
A conventional laser based surveying apparatus developed and currently marketed by Laser Technologies, Inc. of Englewood, Colo. is the Criterion.TM. series of surveying instruments. The Criterion.TM. instrument includes, in one housing, a laser transmitter and receiver, a microprocessor, a numeric keypad for data entry and instrument control commands, an internal flux gate compass for bearing/azimuth measurement, a tilt angle sensor for inclination measurements, an aiming or spotting telescope mounted on the housing, a data collector, and serial port for data transfer. This instrument measures and computes heights, horizontal distances, vertical distances, slope distances, inclinations, coordinates, bearings or azimuths, and multiple point traverse survey data. The instrument may also be coupled to a global positioning system (GPS) receiver to pinpoint the location from which measurements are being taken.
The Criterion.TM. instrument utilizes an external battery pack which typically may be worn on the user's belt or back. The battery pack preferably should be kept at least 12 inches away from the instrument itself to prevent magnetic interference from the battery pack from distorting the internal flux gate compass sensor readings. In addition, the internal location of the compass in the Criterion had to be chosen to be as remote as possible from the unit's circuit boards to minimize the effect of internal circulating electrical currents during system operation on the readings from the flux gate compass.
Conventional laser based survey instruments such as the Criterion, which has an internally mounted flux gate compass, are limited in the inclinations for which azimuth readings may accurately be made. This is because the flux gate compass must be held within .+-.15 degrees of level in order to generate an accurate azimuth signal. This .+-.15 degree requirement is imposed because the core of the compass is a floating ring which must remain fairly level, i.e., within the 15 degree range. In addition, accuracy decreases as one approaches this 15 degree limit. Thus, for most accurate readings, the Criterion instrument must be held as horizontal as possible.
However, many measurement situations arise where the inclination is greater than 15 degrees. In these cases, two separate measurements must be taken, one horizontally for azimuth, and then the instrument repositioned, for inclination or slope distance measurement. Repositioning often leads to variation in the resultant readings because of the separate measurements required and the opportunity to introduce errors.
Another conventional surveying system is marketed by MDL Technologies L.L.C. This system consists of a laser rangefinder with a built in inclinometer and flux gate compass mounted on a conventional hard hat worn by construction workers. The laser rangefinder, flux gate compass, and inclinometer are all referenced to a common axis of alignment. The hard hat wearer looks toward the target and actuates a hand held trigger to take a measurement which is then fed to a hand held data recorder. This arrangement suffers from a similar limitation to that of the Criterion instrument in that accurate compass readings can only be taken when the apparatus is substantially level because of the floating core of the flux gate compass.
Therefore there is a need for a survey apparatus which permits the compass or angle sensor to be positioned independently of the laser based distance measuring instrument, yet retain the real time interconnection between the instrument and the compass and permit simultaneous inclination and azimuth readings at any inclination angle. There is also a need for a system configuration which allows the compass to be physically coupled to or decoupled from the laser based distance measuring apparatus to permit compass operation free from the effects of internally generated circulating electrical currents in the laser-based distance measuring device.
There is also a need for a surveying system that can be uniquely configured for the particular survey tasks at hand. For example, a very precise and expensive laser distance measuring device that has one centimeter resolution at 100 meters is clearly not needed for measurements involving distances more than 1000 meters. Similarly, measurements that do not need relative azimuth angle accuracies do not require the use of a measuring instrument which includes an angle encoder. Similarly, where exact GPS location determinations are not needed in addition to range and inclination, it should not be necessary to provide a GPS receiver. However, when the measuring requirements change, it is therefore desirable to provide the capability to rapidly and conveniently to change the configuration of compass, data logger, and/or laser-based distance measuring apparatus, without having to buy an entirely new system. In addition, there are many situations that may occur which do not require all of these features. Accordingly there is a need for a relatively low cost precision modular surveying measurement system that can be configured in a variety of ways depending on the current surveying requirements and one in which the configuration may be readily changed by combining, adding or removing modules.